1. Field of the Invention
The present invention relates to a battery charging circuit for charging a rechargeable battery, and more particularly, to a battery charging circuit capable of enhancing operating stability.
2. Description of the Prior Art
Rechargeable batteries are widely used in a variety of handheld or mobile electronic devices, such as a mobile phone, a personal digital assistant (PDA), a laptop, etc. Since a rechargeable battery needs to be charged with a corresponding battery charger, quality of a battery charger affects safety, cost and efficiency of charging a rechargeable battery.
In general, lithium batteries are the most widely used rechargeable batteries by consumers. In a normal condition, a voltage across a lithium battery varies within a specific range. In other words, the voltage across the lithium battery is around 4.2V when the lithium battery is fully charged. After normal operations, electric energy stored in the lithium battery is gradually consumed, and the voltage across the lithium battery drops to around 3.0V. As a result, the lithium battery needs to be charged by a battery charger, such that the lithium battery can be refilled with electric energy and provides electric power to electronic devices. Therefore, a basic function of the battery charger provides electric power to a rechargeable battery during charging process, such that the voltage of the rechargeable battery can gradually increase from around 3.0V to around 4.2V, i.e. the rechargeable battery is fully charged. Noticeably, if the voltage across the lithium battery is less than 3.0V, it means the lithium battery may be damaged inside. As a result, the battery charger needs to charge the lithium battery with a trickle current first (known as a trickle mode), and then starts to charge the lithium battery with a greater current until the voltage across the lithium battery is above 3.0V.
In a normal charging operation, i.e. the voltage across the battery is between 3.0V to 4.2V, in order to enhance efficiency and safety of the charging operation, the battery charger charges the rechargeable battery with a greater current first when the electricity of the rechargeable battery is depleted or the voltage across the battery is low (around 3.0V), so as to shorten charging time. The above charging method is known as a constant current (CC) mode. When the voltage across the rechargeable battery approaches a full voltage level (slightly less than 4.2V), the battery charger changes the charging mode to a constant voltage (CV) mode, to charge the rechargeable battery with a constant voltage (4.2V), so as to enhance safety. Therefore, during the charging process, the battery charger can choose CC mode or CV mode according to the voltage across the rechargeable battery, efficiently and safely charge the battery.
Please refer to FIG. 1A, which is a schematic diagram of a conventional constant current battery charging circuit 10. The constant current battery charging circuit 10 includes an error amplifier 100, a low-power transistor 102, a high-power transistor 104 and an external resistor R_ext1 outside a chip. The constant current battery charging circuit 10 can be connected with a rechargeable battery RCBAT1, and charges the rechargeable battery RCBAT1 in the constant current mode. A user can adjust resistance of the external resistor R_ext1 to control a current flowing through the low-power transistor 102. Then, since gate voltages of the low-power transistor 102 and the high-power transistor 104 are the same, and both the low-power transistor 102 and the high-power transistor 104 are P-type metal oxide semiconductor field effect transistors (PMOSFETs), a difference between the low-power transistor 102 and the high-power transistor 104 is that a width to length ratio of the high-power transistor 104 is a multiple of that of the low-power transistor 102. Therefore, the current flowing through the high-power transistor 104 is the multiple of the current flowing through the low-power transistor 102. In other words, the high-power transistor 104 can be conducted with the current equal to the multiple of that of the low-power transistor 102. Therefore, the current flowing through the low-power transistor 102 mirrors many times current in the high-power transistor 104, so as to effectively charge the rechargeable battery RCBAT1.
Furthermore, please refer to FIG. 1B, which is one of conventional constant voltage battery charging circuits 15. The constant voltage battery charging circuit 15 includes an error amplifier 150, a high-power transistor 152, and resistors R1 and R2 for dividing voltage. The constant voltage battery charging circuit 15 fixes an output voltage at a specific voltage, and charges the rechargeable battery RCBAT1 in the constant voltage mode.
As can be seen from the above, the battery charger can choose the constant current mode or the constant voltage mode to charge the rechargeable battery. For simplifying circuits, the high-power transistors 104 and 152 can be realized by the same high-power transistor, and the constant current battery charging circuit 10 and the constant voltage battery charging circuit 15 can be combined into one circuit after a mode determination mechanism is added. Conventionally, the battery charger compares output voltages of the error amplifiers 100 and 150, and then the higher output voltage ties to the gate of high-power transistor, to control conduction of the high-power transistor. As a result, the battery charger can choose a proper mode between the constant current mode and the constant voltage mode to charge the rechargeable battery.
However, since there is a parasitic capacitor C_para1 outside the chip connected with the external resistor R_ext1 in parallel. According to experimental results, once the resistance of the resistor R_ext1 is getting greater, the system oscillates and doesn't apply the constant current, such that the battery charger cannot operate normally.